Within the field of placement of communications and power cables, precise measurement of torque applied to a cable pulling winch is critical to successfully placing a cable. Communications cables are often constructed from fiber optic cable, while power cables are often constructed from various metals. Cable pulling winches ordinarily are utilized to pull a cable through a conduit and utilize a capstan which is driven by a motor. The cable is affixed to a tow rope or line wrapped around the capstan, which rotates thereby pulling the cable. Traditionally, the motors used to pull cables are hydraulically or electrically driven.
The conduits through which the cables are pulled are ordinarily underground. Tension in the cable is a function of numerous factors. A major factor is friction. Friction forces are a function of the length of cable being pulled, the materials from which the cable and conduit are constructed, the number of curves or bends in the conduit, and the amount of added lubrication, among other things. The curves and bends affect the tension a great amount as the friction forces around them is much higher than straight portions of the conduit.
Additionally, as the cable is pulled, the tension throughout the length of the cable is not a constant value. For example, each time the cable passes a curve or bend, friction increases causing a corresponding increase in tension. The increases in tension are cumulative, meaning that the tension in the cable closest to the tow rope is higher than the tension in the cable initially entering the conduit. Further, the tension increases due to the weight of additional cable pulled.
Determining what the tension of the cable will be as a function of each of these factors is an impossible task. However, knowing the tension in the cable is critical to proper placement. As the tension in the cable increases, or if the cable gets stuck in the conduit as it turns or bends, the capstan will continue to pull on the cable thereby increasing the tension within the cable. The increasing tension may possibly lead to a fracture within the cable.
The cumulative affect of increasing the tension is also very important due to the fact that often times the length of the conduit is over one mile. The cumulative nature of the tension values leads to the possibility that the cable may fracture in the last few feet of the conduit. The prior art devices do not provide for accurate and easy measurement of the torque of the capstan or the increasing tensions in the cable.
As can be seen from the fact that fiber optic cables fracture relatively easily, accurate measurement of cable tension, at all times, is critical since over stressing the cable destroys the glass fibers. Some prior art devices have focused on determining the torque on the capstan and correlating that to the tension in the cable. These devices have focused on measuring the hydraulic pressures at the motor inlet. Some prior art devices which use electric motors have focused on measuring amperage. These systems after calibration can determine torque in the capstan by converting the hydraulic pressures, or amperage, into corresponding capstan torque values. However, temperature variations and the range of pressure affect the calibration of such systems. That is, the calibration of such a device changes during operation thus introducing error into the tension readings.
The use of the above-described prior art devices with fiber optic cables leads to inconsistent results. Because a significant calibration problem occurs over time, an apparently proper torque reading may in reality be a dangerously high torque at the capstan. Without any warning of a high torque condition the cable may fracture without the knowledge of the individuals placing it. Depending on the extent of the fracturing the problem may be detected during placement, soon thereafter or even later. Regardless of when it is detected the fractured cable will have to be replaced which involves significant expense. The significant expense would have been avoided if the high torque on the capstan would have been decreased when it became dangerously high.
Other prior art devices provide complicated mechanisms for the determination of torque on a capstan. For example, U.S. Pat. No. 3,360,243 to Betta and U.S. Pat. No. 4,372,535 to Gibson et al. disclose clutch-type mechanisms which measure the forces between two drums to approximate the tension in the cable. Additionally, U.S. Pat. No. 4,048,547 to Havard discloses a mechanism for controlling and adjusting the tension in a cable of mooring winches by measuring the torque exerted on a speed reducer interposed between the capstan and the motor. These devices are cumbersome to operate and introduce complicated components into the apparatus which may fail during operation.
An alternative to measuring the torque at the capstan is placing a load-cell directly in the cable itself, thereby directly measuring the tension in the cable. Typical in-line systems link the load cell between the capstan and the cable. The load cell signal is returned for processing via long conductors in the line. The in-line cell systems are delicate and inconvenient for routine field use.
Therefore, a need has arisen to provide an accurate torque measuring device which solves these and other problems in the prior art.